The invention relates to an electrical circuit for controlling the drive mechanism of a highway crossing gate. Highway crossing gates are currently used at railroad/highway crossings. Such gate mechanisms may also be used at other crossings or areas where it is desired to alert traffic on one right-of-way to the approach of traffic or vehicles on the crossing right-of-way.
Generally these mechanisms use a crossing arm which is counterweighted to a generally horizontal position and which is selectively raised through the use of a drive motor. Such mechanisms can be characterized as those using a two-wire system and those using a three-wire system. In a two-wire system the two wires refer to a control signal which indicates the arm should be in a raised or up position. The second wire in a two-wire system refers to a power wire. In a so-called three-wire system, a control signal indicating a desired arm-position, a control signal indicating a desired arm-down position and a power wire constitute the three wires. In both the two-wire and three-wire systems the nomenclature omits the fact that a common wire is also used.
This invention relates to the type of crossing control known as a two-wire system (actually having at least three wires including a common) which gets an input signal indicating the desirability to have an arm-up position and a power source.
In addition, the highway crossing mechanisms generally use DC drive motors, often through internal gearing, to raise and lower the gate arm. Highway crossing mechanisms can also be characterized by the type of motor used to control both the up and downward movement of the crossing arm, namely permanent magnet motor or field coil motors. This invention relates to a permanent magnet motor type crossing gate control circuit of the two-wire type.
It is the desire in highway crossing gates to maintain a high level of vitality. Vital crossing design is such that component failures are minimized and when such failures do occur, the overall device tends to fail in a default mode. This can be seen in that the highway crossing design generally uses a gravity force to lower the crossing arm. As such, should an input signal fail, the unit can be lowered through the force of gravity to a horizontal or blocking position. To maintain the vital concept throughout the equipment includes the desire to minimize electrical components in the circuit. The reduction of a component from a given circuit reduces the likelihood that such component will fail and increases the overall reliability of the device. Because vital circuits often rely on vital components and vital components can be quite costly, the reduction of circuit components can result in a device that is less costly and more reliable.
Control of the motor in the highway crossing mechanism requires a bidirectional motor operation. Use of separate field coils in DC motors has permitted control circuits to provide the necessary functions using a reduced number of relays. However, prior art devices using permanent magnet motors have required more than one electrical control relay. While it has been desirable to use a permanent magnet motor to obtain the necessary electromechanical drive without the necessity for providing motor field currents, such permanent magnet control circuits have correspondingly required additional circuit control relays. The invention provides for a highway crossing circuit, which controls a permanent magnet motor for both drive-up and drive-down functions of the gate arm, while using only a single control relay. Specific prior art circuits are shown in FIGS. 2-4. Co-pending application, Ser. No. 08/291,094, filed Aug. 16, 1994, of Richard S. Jones et al., entitled Highway Crossing Gate Mechanism Circuit Contact, is incorporated herein by reference.